1. Field of the Invention
This invention generally relates to digital communications, and more specifically relates to the generation of synchronization and data sampling signals from transmitted digital data signals, in particular, non-return-to-zero (NRZ) signals.
2. Description of the Prior Art
Digital communications may require three or more types of synchronization, including: 1) carrier synchronization for determining carrier phase or coherence; 2) bit synchronization for determining one bit interval from another; and 3) word or frame synchronization, which is similar to bit synchronization but used for determining one group of data, i.e., a word or frame, from another.
Bit synchronization, in particular, may be derived from a separate synchronization signal, such as a pilot clock, or from the data signal itself. In currently used digital communication systems, such as GSM (Global Standard for Mobile) communications, the latter method of generating a bit sync signal is generally employed for economic and other reasons. Bit synchronization provides the proper time for sampling data in the received information signal.
A circuit for generating a sync signal directly from an NRZ data signal is illustrated in FIG. 1 of the drawing. An "eye pattern", as shown in FIG. 2, is generally used as an indication of the quality of the digital data received. The eye pattern is a continual, multi-swept display on an oscilloscope of each bit of the data stream. An eye pattern reveals timing errors and distortion in the received data stream. The more "open" the eye of the pattern is, the fewer errors and distortion of the data bit stream exist.
The preferred time for sampling each data bit of the bit stream is where the vertical opening of the eye is the greatest, for example, at time T in FIG. 2. The bit synchronizer of FIG. 1 generates a bit sync pulse signal synchronized to the input data stream and timed to coincide with the maximum eye opening. The data bit stream may then be sampled at the optimum time in correspondence with the bit sync pulse signal.
One of the problems with conventional bit synchronizers and methods for generating a bit sync pulse is an inherent timing inaccuracy in the occurrence of the bit sync pulse. Conventional bit synchronizers, such as the one shown in FIG. 1, generate the bit sync signal by sampling the eye pattern corresponding to the NRZ waveform close to its greatest peak-to-peak voltage or maximum vertical eye opening. At such sampling points, the slope of the NRZ waveform is minimal, with relatively small changes in amplitude, resulting in difficulty in discerning the optimum sampling time for the waveform (at the maximum eye opening).